Electronic photoflash unit

ABSTRACT

Controlled ionization of a photoflash tube by use of a light activated silicon controlled rectifier (LASCR) permits the operator to remain isolated from shock hazard. The LASCR is triggered by a neon lamp which is lit when a camera shutter switch is actuated. The shutter switch and lamp circuit are isolated from the AC line by a low power transformer and from the high voltage DC by the lamp-LASCR arrangement. A bank of capacitors discharges through the ionized photoflash tube and has its capacity rendered variable by a switch in which each switch position corresponds to a respective f-stop in the camera. Flash tube afterglow is eliminated by a relay circuit which prevents the capacitor bank from being recharged by the high voltage supply until the flash tube de-ionizes. Another relay circuit prevents operation of the flash tube prior to full charge of the capacitor bank.

United States Patent [191 Cooper 1 Aug.7, 1973 ELECTRONIC PHOTOFLASH UNIT David 0. Cooper, 2505 N. Quantico St., Arlington, Va. 22207 [22] Filed: Nov. 24, 1971 [2-1] Appl. No.: 201,786

[76] Inventor:

[52] U.S. Cl. 315/241 P, 315/1'49, 315/151,

315/159, 315/241 R [51] Int. Cl. 1105b 37/00 [58] Field of Search ..3l5/l49, 151,159, 315/241 P, 241 R [56] References Cited UNITED STATES PATENTS 3,288,044 11/1966 Bramer..... 315/149 X 3,675,073 4/1972 Hogue 315/149 3,526,821 9/1970 Thomas.... 315/241 R X 3,340,426 9/1967 Elliott 315/241 P X 3,229,158 1/1966 Jensen 315/159 X Primary ExaminerRoy Lake 'i fEiwir rkweese!- D Attorriey-Howard L. Rose and Ira C. Edell [57] ABSTRACT Controlled ionization of a photoflash tube by use of a light activated silicon controlled rectifier (LASCR) permits the operator to remain isolated from shock hazard. The LASCR is triggered by a neon lamp which is lit when a camera shutter switch is actuated. The shutter switch and lamp circuit are isolated from the AC line by a low power transformer and from the high voltage DC by the lamp-LASCR arrangement. A bank of capacitors discharges through the ionized photoflash tube and has its capacity rendered variable by a switch in which each switch position corresponds to a respective fstop in the camera. Flash tube afterglow is eliminated by a relay circuit which prevents the capacitor bank from being recharged bythe high-voltage supply until the flash tube deionizes. Another relay circuit prevents operation of the flash tube prior to full charge of the capacitor bank.

15 Claims, 2 Drawing Figures PATENIEB 3.751.714

IFIGJ F ML INVENTOR. nvu: 0. COOPER QTY 0 IZN EYS ELECTRONIC PHOTOFLASH UNIT BACKGROUND OF THE INVENTION The present invention relates to electronic flash units, and rnoreparticularly to improvements in such units whereby the dutycycle of the unit is improved without the introduction'of electrical shock hazards to the operator.

A problem area in prior art electronic flash units concerns the recovery period of the unit. Specifically, the recovery period, during which a capacitor is charged by a high voltage supply, depends r pen the resistance in the charging circuit. In most prior art units this resis tance includes the equivalent series resistance of a power transformer and a current limiting resistor utilized to protect the power transformer and prevent afterglow of the flash tube. In order to decrease the equivalent resistance of the transformer, the amount of iron in the transformer must be increased, thereby significantly increasing the weight of the unit.

It is therefore an object of the present invention to minimize the recovery period of an electronic photoflash unit without increasing the weight of the unit and without causing afterglow of the flash tube.

Another problem area in prior art electronic flash units relates to the lack of flexibility of the output light intensity of the unit. Specifically, the flash intensity is the same for every flash; therefore it is necessary to adjust the distance between the camera and the subject and to adjust the lens aperture to conform to the constant flash intensity. In some cases, the lens aperture range or camera field of view is not sufficient to permit certain subjects to be adequately photographed. A prior art attempt to correct for this problem is found in U.S. Pat. No. 2,901,67l to Most. This patent describes a photoflash unit in which the voltage applied across the capacitor bank is adjustable as a function of lens aperture. In theory this approach is fine but in practice there are problems. Specifically, the output light intensity varies as the square of the capacitor bank voltage. Also, for f-number settings ranging from 1.4 to 22, the corresponding range of the amount of light required to be admitted into the camera for equal exposure varies from I to 256. Thus, in order ,for the flash brightness to correspond to the f-number setting throughout the entire f-number range, the voltage must vary over a range equivalent to I to TV 256, or! to 16. If it is assumed the extremely low value of 200 volts is the minimum voltage required to cause a flash in an ionized flash tube, the voltage across the capacitor bank must be adjustable over a range of 200 volts to 3,200 volts to conform to the f-number range. To obtain 3,200 volts across the capacitor bank requires a unit which would be too large and heavy to be practical.

It is therefore another object of the present invention toprovide an electronic photoflash unit for which the output light intensity is adjustable with camera fnu mber setting but which does not require unrealistic voltages across the capacitor bank.

SUMMARY OF THE INVENTION According to the present invention the shock hazard of an electronic photoflash unit is eliminated by employing a light activated silicon controlled rectifier (LASCR) in the trigger circuit for the flash tube. The LASCR is turned on by light energy from a lamp, the lamp in turn being part of a shutter switch circuit which is electrically isolated from the LASCR circuit. Input power for the shutter switch and lamp circuit is received from the AC line through an isolation transformer,thereby permitting both sides of the AC line to be maintained floating" relative to chassis ground.

Recovery time of the unit is improved by replacing the power transformer with a lighter rectifier-capacitor voltage multiplier, thereby eliminating the transformer resistance and the need for a current limiting resistor. Afterglow is prevented by a timed relay circuit which operates when the tube is flashed to delay the main capacitor bank from being recharged immediately by'the high voltage supply.

The output light intensity of the unit is adjusted by switching capacitors in and out of the bank of discharge capacitors in accordance with the f-number setting on the camera. By doubling the effective capacity with each switch position, the output light intensity, which varies in proportion to the effective capacity of the bank, is adjustable in increments corresponding to fnumber settings.

BRIEF DESCRIPTION OF THE DRAWINGS The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein: j Y

FIG. I is a schematic circuit diagram of the photoflash unit of the present invention;

FIG. 2 is a schematic diagram illustrating electrical connections for a flash tube to the jacks J5 or J6 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring in detail to the accompanying drawings, a photoflash unit includes a pair of parallel-connected three-wire AC jacks J 1, J2. Jack J1 receives input power for the unit and J2 is a convenience AC outlet. The AC ground wire is connected to chassis ground at the unit; the two active AC lines, however, are maintained floating relative to the chassis. AC power is supplied to the various .operating'circuits through one pole of a double-pole double-throw switch, 81', when the latter is in its on position. Swich S2, connected in series initial surge of power to charge the capacitor bank.

Switch S3 serves to selectively apply AC power to lamp ML which is co-located with the flash tube and may be used for low level lighting during the set-up prior to the use of the high power photoflash unit.

A voltage multiplier circuit, comprising rectifiers CR3, CR4, CR5, CR6, CR7 and CR8, and capacitors C23, C24, C25, C26, C14 and C22 is connected to receive AC power through switches S1 and S2 and provide a DC voltage at a level significantly higher than the peak amplitude of the input AC voltage. Specifically, rectifiers CR3, CR4 and CR5 and capacitor C14 are connected in series between the two active AC lines. Rectifiers CR6, CR7 and CR8 and capacitor C22 are also connected in series between the two AC lines but the latter group of rectifiers are poled opposite to the first-mentioned group of rectifiers. Capacitor C23 is connected across the series combination of CR3 and CR4; capacitor C26 is similarly connected across the series combination of CR6 and CR7. Capacitor C24 is connected across the series combination of CR4, CR and C14; likewise C25 is connected across the series combination of CR7, CR8 and C22.

The voltage multiplier circuit as described comprises two conventional voltage tripler circuits, each substantially of the type described in Handbook of Semiconductor Electronics," third edition, published in 1970 by McGraw Hill, lnc. (Library of Congress Catalog Card No. 6947183 at Section l7.2c. The two voltage tripler circuits are connected in series to provide an effective multiplication factor of six. Thus, if the peak amplitude of the AC input voltage is l50 volts, the total voltage appearing across the series combination ofC14 and C22 is 900 volts DC, there being 450 volts across each of these capacitors. Capacitors C23 and C26 each charge to 300 volts; C24 and C25 each charge to 150 volts.

Capacitors C14 and C22 also constitute part of the main capacitor bank of the unit and as such are arranged to discharge through one or more flash tubes FT when the latter are triggered. The other capacitors in the bank include capacitors C7 through C13, inclusive and C15 through C21, inclusive. However, these other capacltors are permitted to discharge through flash tubes FT only when brightness controll switch S5 is appropriately positioned. Specifically, capacitor C13 is connected in series with resistor R21 and rectifer CR11, the series combination being connected across C14. Likewise, C21 is connected in series with resistor R77 and rectifier CR1, the series combination being connected across C22. Rectifiers CR11 and CR14 are poled to permit C13 and C21 to be charged by the voltage multiplier circuit whenever C14 and C22 are so charged; however, these rectiflers are poled to block discharge current through the flash tubes FT. Switch S5 is a three-pole (A, B, C), four position (1, 2, 3, 4) switch. Pole C connects a short circuit across R27, CR14, CR11 and R32 for positions 2, 3 and 4 of the switch. thus, when S5 is in any of these positions, the effect of CR11 and CR14 is eliminated and C13 and C21 are permitted to discharge through the flash tubes along with C14 and C22.

A similar arrangement is provided for the other capacitors in the bank, all of which are capable of being switched into the discharge circuit whereby their respective rectifiers and charging resistances are shorted out. Thus, in positions 3 and 4 of switch S5, pole B of that switch shorts out the series combination of R19, CR10, CR13 and R25 to place C11, C12 and C19, C in the discharge circuit. Pole A of S5 is operative in position 4 to short out R17, CR9, CR12 and R23 to place C7, C8, C9, C10 and C15, C16, C17, C18 in the discharge circuit. In position 1 of S5 only C14 and C22 are in the discharge circuit.

If, as assumed here, all of capacitors C7 through C22 have the same capacitance, switching of S5 from position 1 to position 2 doubles the capacitance of the discharge circuit. The capacitance of this circuit is doubled again in going from postion 2 to position 3, and doubled again in going from position 3 to position 4. Since all of the capacitors in the bank are charged to the same voltage, the light intensity provided by flash tubes FT is proportional to the capacitance of the discharge circuit. Since this capacitance is variable by S5 by powers of two, the output light intensity is similarly varied by 55. Since consecutive f-stop settings on a camera vary the admitted light by powers of 2, switch S5 permits the output intensity to be varied to accommodate desired f-stop settings. As a practical matter, S5 may be linked to the f-stop adjustment on the camera so that both the f-stop setting an output light intensity may be varied with a single adjustment. While only four capacitance settings are described above, additional settings may be provided by adding additional capacitors and additional poles and positions for S5.

It is to be noted that resistor R20 is connected in parallel across R21 and CR11. Likewise R26 is connected across R14 and R27; R18 is connected across R19 and CR10; R24 is connected across R25 and CR13; R16 is connected across R17 and CR9; and R22 is connected across CR12 and R24. Resistors R20, R26, R18, R24, R16, and R22 have relatively high resistance, at least an order of magnitude greater than the resistances connected in series with the charging circuit rectifiers. Consequently, these large resistors do not conduct a significant amount of discharge current through the flash tubes FT. Thus, for example, with S5 in position 1, R20 and R26 permit C13 and C21 to discharge very slightly through the flash tubes FT when C14 and C22 so discharge, but the discharge contribution of C13 and C21 is made almost negligible. This contribution is so low as not to c'ause afterglow of the flash tube once C14 and C22 have discharged completely. The primary function of large resistors R20, etc. is to permit gradual discharge of the capacitor bank when the unit is turned off. Thus, when S1 is switched to the off position, one pole removes AC power from the voltage multiplier circuit. The other pole connects a relatively low resistance R1 across the capacitor bank for the purpose of discharging the latter. lf R20, etc. were not present, those capacitors not switched into the discharge circuit by S5 could not discharge. Thus, for example, if S5 is in position 1, when the unit is turned off C13 and C21 discharge gradually through R20, R26 and R1. For practical values of these resistances and of the capacitors in the bank, the gradual discharge takes less tha l minute. of course those capacitors which are in the discharge circuit discharge much more quickly through R1 alone, the latter being considerably smaller in resistance value than R20, etc.

The capacitor bank cannot discharge through flash tubes FT until the tubes are triggered. The circuitry for triggering the flash tubes is described in the following paragraphs.

An isolation transformer T2 has a primary winding connected across the two active AC lines as controlled by switches S1 and S2. The resulting voltage appearing across the secondary winding of T2 is rectified by CR2 and filtered by C2 to provide a DC voltage. Resistors R2, R4 and R3 are connected in series across C2 and fonn a voltage divider. The resistance of R3 is more than an order of magnitude greater than that of R2 and R4. A charging capacitor C3 is connected across the series combination of R3 and R4; a neon lamp NE-l and current limiting resistor R5 are connected in series across R4. One side of each of C2, C3, R3 and the secondary winding of T2 is connected to chassis ground. The camera shutter switch (not illustrated) is connected to pins 4 and 5 of jack J3 and in parallel with single-pole single-throw normally open push-button test switch S4. These parallel switches are connected in series with a pair of normally open contacts of a relay RL2, the series combination being connected across R3.

One of the active AC lines is also the electrical common point of the circuit. The voltage between this point and the other active AC line as controlled by S1 and S2 is rectified by CR1 and filtered by C1 to provide a low negative DC voltage supply of about 150 volts. Resisto the electrical common point and its cathode is connected to the junction between R7 and R8 so that R7 is connected between the anode and cathode of LASCR- 1. Resistor R6 is connected between the gate and cathode of LASCRJ The cathode of LASCR-l is also connected to pin-B ofjacks J5 and J6. Pin H of J5 and J6 is connected'to the electrical common point. Connected between pins 13 and H of these jacks is a capacitor C27 connected in series with the primary windingof a transformer T3. When LASCR-l is not conducting, C27 charges to the polarity shown in FIG. 2. When LASCR-l is activated by lightfrom NE-l it provides a short circuit between pins B and H permitting C27 to discharge through the primary winding of T3 causing a pulse to appear at the secondary which is applied to the trigger electrode of flash tube FT to ionize the tube and permit the capacitor bank to discharge therethrough.

The winding of a control relay RL-l is connected in series with capacitor C4 across the anode and cathode of LASCR-l. Relay RL-l is a two-pole relay, one pole controlling application of AC power to the voltage multiplier circuit; that is, when RL-l is energized, one of the active AC power lines to the voltage multiplier is interrupted. The other pole of RL-l includes a contact is about 10 volts each. It now test switch S4 or the shutter switch is momentarily closed, R3 is shorted out of the circuit. C3, which had previously charged to approximately 140 volts, now discharges through actuator lamp NE-] and current limiting resistor R5. The resulting light flash from NE-l activates LASCR-l as described below. If R3 remains shorted by S4 or the shutter switch, C3, after flashing NE-l, re-charges through R2 to the voltage across R4, which would be approximately 75 volts with R3 out of the circuit. This would be too low to produce a bright glow in NE-l so that LASCR-l would not be continuously activated. When R3 is returned to the circuit after S4 or the shutter switch is released, C3 re-charges to 140 volts and is ready for the next activation.

Prior to activation of LASCR-l, C4 and C27 (FIG. 2) are charged to approximately 130 volts with the polarities shown. Flashing of NE-l causes LASCR-l to arm connected in series with capacitor C5 which is returned to the electrical common point. The normally open contact of the second poleof RL-l is connected via resistor R10 to the junction between the. winding of RL-l-and C4. The normally closed contact of the second pole of RL-1 isconnectedvia resistor R11 to the negative terminal of the low DC voltage supply providinga normal negative charge of abou t 150 volts on C5.

One pole of RL-2, when RL-2 is energized, connects push-button test switch S4 (and the paralleled shutter switch, not shown, connected to pins 4 and 5 of jack J3) across R3 in the secondary circuit of T2. The winding of R L-Z is connected in series with resistor R12 and potentiometer R28 across one half of the voltage multiplier circuit and remains closed as long as the capacitor bank is charged to at least 90 percent of its rated final charge. R28 is adjusted to assure that RL-2 pulls in at this 90 percent level.

In operation, assume the unit tojbe in standby condition with AC power applied to the voltage multiplier and the capacitor bank charged. RL-2 is energized in this condition and RL-l is de-energized. Because of the relative resistances of R2, R3 and R4, if C2 is charged to 150 volts D.C., the voltage across R3 is approximately 130 volts whereas ,the voltage across R2 and R4 theprimary winding of T3 and ionize the gas in flash tube FT. The flash tube is thereby rendered conductive and the active capacitors (i.e., those connected in the discharge circuit by S5) discharge through the tube. There is no practical time lag between flashing of NE-l and flashing of FT so that the unit'is capable of proper synchronization with cameras having very high shutter speeds.

When LASCR-l is activated, C4 also discharges through it and the winding of RL-l which becomes mo mentarily energized thereby. This opens the AC supply line to the voltage multiplier and also connects C5 to the winding of RL-l through R10. The voltage thus applied by C5 to the winding of RL-l and across LASCR- 1 is opposite in polarity and of greater magnitude than that just previously applied by C4 due to the activation of LASCR-l. Therefore, the leading edge of the voltage transition created by C5 cuts of LASCR-l to render it non-conducting. C5 then discharges through the winding of RL-l, holding the latter energized and preventing application of AC power to the voltage multiplier. The duration of the period of energization of RL-l is determined by the time'constant of R10 (plus the resistance of the coil-of RL-l) and C5. This time constant is made long enough to permit flash tube F1 to deionize before AC power is re-applied to the voltage multiplier; in this manner afterglow of the flash'tube is prevented. importantly, this time constant is made no longer than necessary to prevent afterglow because it addsto recovery time between photoflashes of the unit.

One terminal each of the ready light NE-Z, C6 and R12are connected through R13 to the normally open contact of the second pole of R12. The associated contact arm is connected to the electrical common'point. The other terminals of C and R14 are connected together and to one terminal of J4. With a bell plugged into J4, C6 and R14 are connected through the coil winding of the bell to the same normally open contact of R12. When the capacitor bank charges to of full voltage, R12 becomes energized, the normally open contact closes completely the circuit to light NE2 and charge C6. The pulse of charging current actuates one ring of the bell to indicate the readiness of the unit. When the capacitor bank discharges through the flash tube FT, the high output voltage of the voltage multiplier drops to a very low value and relay RL-2 is deenergized, turning off lamp NE-Z and permitting C6 to discharge through R14.

Connection of S4 and the shutter switch in series with one pole of relay RL-2 prevents actuation of the unit before the capacitor bank has recharged. Specifically, since RL-Z is deenergized only between flash of FT and attainment of 90 percent of full recharge, the shutter switch is prevented from triggering the flash tube during that period.

A second light activated silicon controlled rectifier LASCR-2 may be plugged into the unit directly at pins 1, 2 and 3 of J3 or located remotely and connected to J3 by cable. This would be substituted for a cable to the camera shutter switch. LASCR-2 is adapted to permit triggering of the unit by an external light flash so that two or more units may be synchronized. LASCR-2 is connected substantially in parallel with LASCR-l; the parallel circuit comprising R and indicator L in the gate circuit of LASCR-2 prevents the latter from operating in the presence of strong steady light. Rather, only a sudden pulse of light directed at LASCR-2 causes it to conduct and thereby permit C27 to trigger flash tube FT.

it will be appreciated that the possibility of shock to the operator is eliminated in the unit by maintaining both active AC lines floating and connecting the ground terminal of J1 and J2 to chassis. The electrical common point of the circuit, therefore, which is one of the active AC lines, is insulated from chassis. Electrical connection to the shutter switch and S4 is isolated from high voltage by means of the secondary circuit of T2 which is isolated at its input by T2 and at its output by the NE-l/LASCR-l arrangement.

The recovery period of the unit (i.e., the minimum time between flashes) is minimized by the minimization of resistance in the charging circuit for the capacitor bank. Specifically, no power transformer or current limiting resistor is required in the unit as described; the equivalent series charging resistance is merely the extremely low resistance inherent in the rectifiers, capacitors and the connecting leads. Afterglow is nonetheless prevented by keeping RL-l energized to prevent charging of the bank until flash tube FT de-ionizes.

Substantially all of the circuitry described is preferably located inside the case or other suitable enclosure provided for the unit. Exceptions are the flash tube FT, LASCR-Z, NE-Z and the various switch actuators.

By way of example only, the following table indicates typical values for the components utilized in the circuits of FIGS. 1 and 2:

Rl-'200 ohms R2-3.3K ohms R3 -47K ohms R43.3K ohms R5 100 ohms R6-56K ohms R7-l20K ohms R8-390 ohms R9-22K ohms RIO-470 ohms Rl l-75 ohms Rl2-33K ohms Rl3-1OOK ohms R1433K ohms Rl533K ohms Rl62.5K ohms Rl7-250 ohms Rl82.5l( ohms Rl9-250 ohms R205K ohms R2l-250 ohms R222.5K ohms R23250 ohms R24-2.5K ohms R25250--ohms R265K ohms R27--250 ohms R28-5K ohms Cl-300 uf C2l0 uf C3-10 uf C4-8 uf C5-80 uf C66 uf C7 through C26, each-525 uf C 27-l.0 uf

The values listed above provide a one second recovery time between flashes; that is, the capacitor bank is charged to percent of full value in about one second. Recovery time can be speeded up by increasing the capacitance in the voltage multiplier circuit.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and sgope of the invention as definedin the appended claims.

I claim:

1. An electronic photoflash unit comprising:

means for applying AC power to said unit;

a triggerable flash tube containing ionizable gas which is ionized when said tube is triggered; selectively actuable triggering means for at will triggering said flash tube;

power supply means for converting said AC power to a relatively high DC voltage and applying said relatively high DC voltage across'said flash tube, said power supply means including:

a capacitor-rectifier voltage multiplier for providing said relatively high DC voltage; and a bank of capacitors connected to be charged by said voltage multiplier and to discharee through said flash tube when said flash tube is triggered;

control means responsive to actuation of said triggering means for removing AC power from said voltage multiplier circuit for a predetermined period of time at least equal to the time required for gas in the triggered flash tube to de-ionize.

2. The unit according to claim 1 further comprising means responsive to the voltage across said bank of capacitors for preventing triggering of said flash tube after discharge of said capacitors throuh said flash tube until said capacitors re-charge to approximately 90 percent of said relatively high DC voltage.

3. The unit according to claim 2 wherein said bank of capacitors includes means for selectively varying the capacity capable of discharging through said flash tube, said capacity being variable in discrete steps related by powers of two.

4. The unit according to claim 1 wherein said control means includes a relay having a winding and at least one pair of normally closed contacts through which said AC power is applied to said voltage multiplier, said control means further comprising: means responsive to triggering of said flash tube for energizing said relay winding to open said normally closed contacts; and resistive-capacitive means for holding said relay winding energized for a period corresponding to the time required for said gas to de-ionize.

5. An electronic photoflash unit of the type wherein a predetermined stored electrical charge is selectively discharged through a triggerable flash tube in response to actuation of manually-actuable switching means, said photoflash unit being characterized by a trigger circuit which is responsive to said manually-actuable switching means for discharging said predetermined stored charge through said flash tube and which electrically isolates said manually-actuable switching means from said predetermined stored charge, said trigger circuit comprising:

a light-actuated switching element of the type which is rendered conductive to electrical current when impinged upon by light of at least predetermined intensity, said switching element being nonconductive when not impinged upon by light of at least said predetermined intensity;

means responsive to current conduction through said light-actuated switching element for triggering said flash tube to discharge said predetermined stored charge through said flash tube;

a lamp circuit electrically isolated from said light actuated switching element and said predetermined stored charge and including:

a lamp arranged to emit light to impinge upon said light-actuated switching element when said lamp is energized;

voltage supply means for said lamp; and

circuit means forapplying energizing voltage from said voltage supply means to said lamp in response to actuation of said manually-actuable means, said energizing voltage being sufficient to cause light from said lamp to impinge upon said light-actuated switching element at least at said predetermined intensity.

6. The unit according to claim wherein said stored electrical charge is stored in a bank of capacitors, and further comprising power supply means for charging said bank of capacitors to said'predetermined charge.

7. The unit according to claim 6 further comprising output light intensity adjustment means including switches to permit'selective switching of capacitors of said bank'into and out of active status, said switches and capacitors being arranged such that the total active capacity in said bank of capacitors is adjustable in discrete powers of two.

8. The unit according to claim 7 further comprising a plurality of diodes, each connected in series with a respective capacitor in said bank, said diodes being poled to block discharge current flow through said capacitors, and wherein said switches in said output light intensity adjustment means are arranged to selectively short-circuit said diodes to permit discharge currentto flow through selected capacitors in said bank.

9. The unit according to claim 6 further comprising means responsive to actuation of said light-actuated switching element for inhibiting charging of said bank of capacitors by said power supply means for a predetermined period of time, said predetermined period of time being longer than the time required for gases in said flash tube after de-ionize after discharge.

10. The unit according to claim 6 further comprising means responsive to the charge stored in said bank of capacitors for inhibiting application of said energizing voltage to said lamp unless the charge stored in said bank of capacitors is equal to at least percent of said predetermined charge.

11. The unit acording to claim 6 wherein said power supply means comprises:

means for applying AC voltage to said unit; voltage multiplier means for converting said AC voltage to a relatively high DC voltage; and means for applying said relatively high DC voltage to said bank of capacitors to provide said stored charge.

12. The unit according to claim 11 wherein said voltage supply means comprises rectifier and filter means for converting said AC voltage to a relatively low DC voltage, and wherein said circuit means comprises:

a storage capacitor; I

a'charging path for applying said relatively low DC voltage across said storage capacitor;

a manually-'actuable discharge path for said capacitor, including said manually-actuable means and said lamp, for selectively discharging said-storage capacitor through said lamp.

13. The unit according to claim 12 wherein said manually-actuable means comprises a switch having short and open circuit positions, and wherein said discharge path further comprises:

first and second resistances connected in series across said storage capacitor, said switch being connected in parallel with said second resistance;

a current-limiting resistance connected in series with said lamp, the series combination of said lamp and current-limiting resistance being connected in parallel with said first resistance.

14. The unit according to claim 5 further comprising means responsive to actuation of said light-actuated switching element for inhibiting storage of said predetermined charge for a predetermined period of time, said predetermined period of time being at least as long as the time required for gases in said flash tube to deionize after discharge.

15. The unit according to claim 5 further comprising means responsive to said stored electrical charge for inhibiting application of said energizing voltage to said lamp unless the stored electrical charge is equal to at least 90% of said predetermined stored charge. 

1. An electronic photoflash unit comprising: means for applying AC power to said unit; a triggerable flash tube containing ionizable gas which is ionized when said tube is triggered; selectively actuable triggering means for at will triggering said flash tube; power supply means for converting said AC power to a relatively high DC voltage and applying said relatively high DC voltage across said flash tube, said power supply means including: a capacitor-rectifier voltage multiplier for providing said relatively high DC voltage; and a bank of capacitors connected to be charged by said voltage multiplier and to discharee through said flash tube when said flash tube is triggered; control means responsive to actuation of said triggering means for removing AC power from said voltage multiplier circuit for a predetermined period of time at least equal to the time required for gas in the triggered flash tube to de-ionize.
 2. The unit according to claim 1 further comprising means responsive to the voltage across said bank of capacitors for preventing triggering of said flash tube after discharge of said capacitors through said flash tube until said capacitors re-charge to approximately 90 percent of said relatively high DC voltage.
 3. The unit according to claim 2 wherein said bank of capacitors includes means for selectively varying the capacity capable of discharging through said flash tube, said capacity being variable in discrete steps related by powers of two.
 4. The unit according to claim 1 wherein said control means includes a relay having a winding and at least one pair of normally closed contacts through which said AC power is applied to said voltage multiplier, said control means further comprising: means responsive to triggering of said flash tube for energizing said relay winding to open said normally closed contacts; and resistive-capacitive means for holding said relay winding energized for a period corresponding to the time required for said gas to de-ionize.
 5. An electronic photoflash unit of the type wherein a predetermined stored electrical charge is selectively discharged through a triggerable flash tube in response to actuation of manually-actuable switching means, said photoflash unit being characterized by a trigger circuit which is responsive to said manually-actuable switching means for discharging said predetermined stored charge through said flash tube and which electrically isolates said manually-actuable switching means from said predetermined stored charge, said trigger circuit comprising: a light-actuated switching element of the type which is rendered conductive to electrical current when impinged upon by light of at least predetermined intensity, said switching element being non-conductive when not impinged upon by light of at least said predetermined intensity; means responsive to current conduction through said light-actuated switching element for triggering said flash tube to discharge said predetermined stored charge through said flash tube; a lamp circuit electrically isolated from said light actuated switching element and said predetermined stored charge and including: a lamp arranged to emit light to impinge upon said light-actuated switching element when said lamp is energized; voltage supply means for said lamp; and circuit means for applying energizing voltage from said voltage supply means to said lamp in response to actuation of said manually-actuable means, said energizing voltage being sufficient to cause light from said lamp to impinge upon said light-actuated switching element at least at said predetermined intensity.
 6. The unit according to claim 5 wherein said stored electrical charge is stored in a bank of capacitors, and further comprising power supply means for charging said bank of capacitors to said predetermined charge.
 7. The unit according to claim 6 further comprising output light intensity adjustment means including switches to permit selective switching of capacitors of said bank into and out of active status, said switches and capacitors being arranged such that the total active capacity in said bank of capacitors is adjustable in discrete powers of two.
 8. The unit according to claim 7 further comprising a plurality of diodes, each connected in series with a respective capacitor in said bank, said diodes being poled to block discharge current flow through said capacitors, and wherein said switches in said output light intensity adjustment means are arranged to selectively short-circuit said diodes to permit discharge current to flow through selected capacitors in said bank.
 9. The unit according to claim 6 further comprising means responsive to actuation of said light-actuated switching element for inhibiting charging of said bank of capacitors by said power supply means for a predetermined period of time, said predetermined period of time being longer than the time required for gases in said flash tube to de-ionize after discharge.
 10. The unit according to claim 6 further comprising means responsive to the charge stored in said bank of capacitors for inhibiting application of said energizing voltage to said lamp unless the charge stored in said bank of capacitors is equal to at least 90 percent of said predetermined charge.
 11. The unit acording to claim 6 wherein said power supply means comprises: means for applying AC voltage to said unit; voltage multiplier means for converting said AC voltage to a relatively high DC voltage; and means for applying said relatively high DC voltage to said bank of capacitors to provide said stored charge.
 12. The unit according to claim 11 wherein said voltage supply means comprises rectifier and filter means for converting said AC voltage to a relatively low DC voltage, and wherein said circuit means comprises: a storage capacitor; a charging path for applying said relatively low DC voltage across said storage capacitor; a manually-actuable discharge path for said capacitor, including said manually-actuable means and said lamp, for selectively discharging said storage capacitor through said lamp.
 13. The unit according to claim 12 wherein said manually-actuable means comprises a switch having short and open circuit positions, and wherein said discharge path further comprises: first and second resistances connected in series across said storage capacitor, said switch being connected in parallel with said second resistance; a current-limiting resistance connected in series with said lamp, the series combination of said lamp and current-limiting resistance being connected in parallel with said first resistance.
 14. The unit according to claim 5 further comprising means responsive to actuation of said light-actuated switching element for inhibiting storage of said predetermined charge for a predetermined period of time, said predetermined period of time being at least as long as the time required for gases in said flash tube to de-ionize after discharge.
 15. The unit according to claim 5 further comprising means responsive to said stored electrical charge for inhibiting application of said energizing voltage to said lamp unless the stored electrical charge is equal to at least 90% of said predetermined stored charge. 